1. Field of the Invention
The present invention relates to a metal-air battery or fuel cell with a long operating life. In particular, this invention relates to a multiple-cell battery or fuel cell with the constituent metal-air cells being activated in a programmed-timing manner to achieve an extended operating life and better utilization of the capacity of individual cells.
2. Brief Description of the Prior Art
Metal-air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having one surface exposed to the atmosphere and another surface exposed to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while anode metal is oxidized, providing a usable electric current that flows through an external circuit connected between the anode and the cathode. The cathode must be permeable to air or oxygen but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuit can be connected. Commercial air cathodes typically contain active carbon, a finely divided hydrophobic polymeric material, a dissociation-promoting catalyst, and a metal screen as the conductive element. A variety of anode metals have been used or proposed for use in metal-air cells. However, zinc, lithium, aluminum, magnesium and alloys of these elements are considered especially advantageous due to their low cost, light weight, and ability to work with a variety of electrolytes.
For instance, a typical aluminum-air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte. There are two basic types of aqueous electrolytes for metal-air batteries: a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In neutral-pH electrolyte, the cell discharge reaction may be written as:4Al+3O2+6H2O→4Al(OH)3 (solid)In alkaline electrolyte, the cell discharge reaction may be written as:4Al+3O2+6H2O+4KOH→4Al(OH)4−+4K+ (liquid solution),which is followed, after the dissolved potassium (or sodium) aluminate exceeds a saturation level, by:4Al(OH)4−+4K+→4Al(OH)3 (solid)+4KOHIn addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows:2A1+6H2O→2Al(OH)3+3H2 (gas).The above equations and similar equations for other types of metal-air cells indicate that it is critically important to regulate the ingress of oxygen and the timing at which electrolyte is brought in contact with the anode. Once oxygen is admitted into a metal-air cell and the anode is in contact with an electrolyte, discharge reactions (including current leakage), anode passivation, and electrode corrosion could proceed regardless if the cell is being used or not to power an external device. This effectively reduces the useful battery life and makes inefficient use of the anode material.
Specifically, state-of-the-art metal-air batteries have been found to exhibit the following shortcomings:    (1) Severe “anode passivation” problem: When the battery is run under high load, large amounts of aluminum hydroxide accumulate on the aluminum anode surface blocking the further access of anode by the electrolyte. In the case of zinc-air cells, zinc oxide layers prevent further access of zinc anode by the electrolyte. Such an anode passivation phenomenon tends to prevent the remaining anode active material from contacting the electrolyte since the remaining anode material is now effectively coated or surrounded by a ceramic layer. Consequently, the electron-generating function ceases and the remaining active anode material can no longer be used (hence, a low-utilization anode). All metal anodes used in state-of-the-art metal-air batteries are known to suffer from the anode passivation problem to varying degrees.    (2) Severe self-discharge and current leakage problems: “Self-discharge” is due to a chemical reaction within a battery that does not provide a usable electric current. Self-discharge diminishes the capacity of a battery for providing a usable electric current. For the case of a metal-air battery, self-discharge occurs, for example, when a metal-air cell dries out and the metal anode is oxidized by the oxygen that seeps into the battery during periods of non-use. Leakage current can be characterized as the electric current that is supplied to a closed circuit by a metal-air cell even when air is not continuously provided to the cell. These problems also result in a low-utilization anode.    (3) Severe corrosion problem: Four metals have been studied extensively for use in metal-air battery systems: zinc (Zn), aluminum (Al), magnesium (Mg), and lithium (Li). Despite the fact that metals such as Al, Mg, and Li have a much higher energy density than zinc, the three metals (Al, Mg, and Li) suffer from severe corrosion problems during storage. Hence, Mg-air and Al-air cells are generally operated either as “reserve” batteries in which the electrolyte solution is added to the cell only when it is decided to begin the discharge, or as “mechanically rechargeable” batteries which have replacement anode units available. The presence of oxygen tends to aggravate the corrosion problem. Since the serious corrosion problem of Zn can be more readily inhibited, Zn-air batteries have been the only commercially viable metal-air systems. It is a great pity that high power or energy density metals like Al, Mg and Li have not been extensively used in a primary or secondary cell.
There is a need for a battery that can be used as an emergency power source at locations where electric supply lines do not exist. Such a battery must have a high energy capacity and a high power density and be capable of running for a long period of time under high load. There is also a need for a battery or fuel cell that can provide a much extended “talk time” and “stand-by” time for a mobile phone. A need also exists for a battery that can power a notebook computer for a much longer period of time (e.g., 12 hours being needed to last for a trans-Pacific flight). Due to their high energy-to-weight ratio, safety of use, and other advantages, metal-air, and particularly zinc-air, batteries have been proposed as a preferred energy source for use in electrically-powered vehicles. However, just like aluminum-air cells, zinc-air batteries also suffer from the problem of “passivation”, in this case, by the formation of a zinc oxide layer that prevents the remaining anode active material (Zn) from contacting the electrolyte.
A number of techniques have been proposed to prevent degradation of battery performance caused by zinc oxide passivation or to somehow extend the operating life of a metal-air battery. In one technique, a sufficient (usually an excessive) amount of electrolyte was added to allow most of the zinc to dissolve (to become Zn ion and thereby giving up the desired electrons). The large amount of electrolyte added significantly increased the total weight of the battery system and, thereby, compromising the specific energy density (energy per unit weight).
In a second approach, anodes were made by compacting powdered zinc onto brass current collectors to form a porous mass with a high surface/volume ratio. In this configuration, the oxide would not significantly block further oxidation of the zinc, provided that the zinc particles were sufficiently small. With excessively small zinc particles, however, zinc was rapidly consumed due to self-discharge and leakage (regardless if the battery is in use or not) and even more serious corrosion problems and, hence, the battery will not last long.
In a third approach, particularly for the development of metal-air batteries as a main power source for vehicle propulsion, focus has been placed on “mechanically rechargeable” primary battery systems. Such a system normally comprises a consumable metal anode and a non-consumable air cathode, with the metal anode being configured to be replaceable once the metal component therein is expended following oxidation in the current-producing reaction. These systems constituted an advance over the previously-proposed secondary battery systems, which have to be electrically charged for an extended period of time once exhausted, and require an external source of direct current.
Most of these mechanically rechargeable systems are quite complex in construction. For instance, the system disclosed in U.S. Pat. No. 4,139,679 (Feb. 13, 1979 to A. Appelby, et al.) contains an active particulate metal anode component freely suspended in an alkaline electrolyte, and a pump to keep the particulate metal anode in suspension and circulated between air cathodes. After discharge of the metal anode component, the electrolyte is then replaced with an electrolyte containing a fresh particulate metal anode component in suspension.
Mechanically rechargeable metal-air batteries with mechanically replaceable anodes have been further developed, e.g., in U.S. Pat. No. 5,196,275 (Mar. 23, 1993 to Goldman, et al.); U.S. Pat. No. 5,318,861 (Jun. 7, 1994 to Harats, et al.); and U.S. Pat. No. 5,418,080 (May 23, 1995 to Korall, et al.). These systems have been designed particularly for use in electric vehicle propulsion, since they facilitate quick recharging of the vehicle batteries simply by replacing the spent anodes, while keeping the air cathodes and other battery structures in place. This mechanical recharging, or refueling, may be accomplished in service stations dedicated to that purpose. However, it is necessary to provide metal-air battery cells that will repeatedly allow insertion and removal of the zinc anode elements for each charge/discharge cycle without causing wear and tear to the mechanically sensitive air electrode flanking each zinc anode.
Another approach to extending the discharge life of a metal-air battery is the “variable-area dynamic anode” method proposed by Faris (e.g., U.S. Pat. No. 5,250,370, Oct. 5, 1993). Such a battery structure includes electrodes, which are moved relative to each other during operation. The electrodes also have areas that are both different in size, with ratios that are variable. The battery structure includes a first electrode, which is fixed in a container. A second electrode is moved past the fixed electrode in the container and battery action such as discharge occurs between proximate areas of the first and second electrodes. A third electrode may be provided in the container to recharge the second electrode as areas of the second electrode are moved past the third electrode at the same time that other areas of the second electrode are being discharged at the first electrode. The ratio of the third electrode area to the first electrode area is much larger than 1, resulting in a recharge time that is much faster, thereby improving the recharge speed. However, this battery structure is very complicated and its operation could present a reliability problem.
Attempts to extend the operating life of a metal-air battery also include the utilization of a deferred actuated battery system, e.g., B. Rao, et al. (U.S. Pat. No. 4,910,102, Mar. 20, 1990; U.S. Pat. No. 5,116,695, May 26, 1992; U.S. Pat. No. 5,166,011, Nov. 24, 1992, and U.S. Pat. No. 5,225,291, Jul. 6, 1993) and J. Ruch, et al. (U.S. Pat. No. 4,490,443, Dec. 25, 1984). Intermittent transfer of electrolyte between cells and a reservoir was proposed by Flanagan (U.S. Pat. No. 5,472,803). The above-cited batteries proposed by Rao, et al. and by Flanagan have the following drawbacks: These batteries involve the operation of a complicated electrolyte delivery system. Further, the deferred actuated battery system proposed by Rao, et al. relied upon a manual actuation operation, not a programmed-timing one. No predetermined criterion or logic was employed to automatically determine if and when a cell should be actuated. The system was based on a single cell or a single cell assembly. Actuation was effected by introducing electrolyte into the anode chamber (not in a programmed fashion and not carried out in an automated manner). Furthermore, no “air admittance on demand” concept was utilized in these batteries; the cell was exposed to the outside air at all times.
In U.S. Pat. No. 5,691,074 (Nov. 25, 1997), Pedicini proposed a diffusion-controlled air vent containing isolating passageways that function to limit the amount of oxygen that can reach the oxygen electrodes when the fan is off and the internal humidity level is relatively constant. This isolation reduces the self-discharge and leakage or drain current of the metal-air cells. In U.S. Pat. No. 5,569,551 (Oct. 29, 1996) and U.S. Pat. No. 5,639,568 (Jun. 17, 1997), Pedicini, et al. proposed the use of an anode bag to limit self-discharge of the cell in an attempt to maintain the capacity of the cell. It was stated that, by wrapping the anode in a micro-porous membrane that is gas-impermeable and liquid-permeable, oxygen from the ambient air that has seeped into the cell must go through a solubility step before it can pass through the anode bag to contact and discharge the anode. However, this solubility step is often not a slow step particularly when the oxygen or air ingress rate into the cell is high. This anode bag provides only a moderately effective approach to reducing the self-discharge problem. This is achieved at the expense of making the cell structure very complicated.
A “restricted gas passageway” concept was proposed much earlier by Przybyla, et al. (U.S. Pat. No. 4,118,544, Oct. 3, 1978) to restrict gas access to the cathode by way of a very small aperture in the cell container, or an additional barrier layer placed within the layer. Oxygen diffusion-limiting membrane was used by Cretzmeyer, et al. (U.S. Pat. No. 4,189,526, Feb. 19, 1980) to improve the active life of a metal-oxygen cell. Several attempts were made to employ a switch or valve to regulate the flow of oxygen into a metal-air cell. Examples include U.S. Pat. No. 4,262,062 (Apr. 14, 1981 to Zatsky), U.S. Pat. No. 4,620,111 (Oct. 28, 1986 to McArthur, et al.), U.S. Pat. No. 5,191,274 (Mar. 2, 1993 to Lloyd, et al.), U.S. Pat. No. 5,069,986 (Dec. 3, 1991 to Dwaorkin, et al.), and U.S. Pat. No. 4,913,983 (Apr. 3, 1990 to Cheiky). Mathews, et al. (U.S. Pat. No. 4,177,327, Dec. 4, 1979) also recognized the importance of intermittently switching on/off an air vent to a metal-air battery for an improved operating life. An electrical actuator is effected to open the air vent only when the battery is supplying electric power to a load. In this manner, the battery is open to the possibility of harsh ambient conditions such as very high or very low ambient relative humidity, prolonged carbon dioxide exposure, etc. However, in the batteries proposed by Mathews, et al. and others cited above, a switch or valve must be manually operated to turn on and off an air access vent and the timing at which this on/off operation is carried out must be determined by the user of the external device. Quite often, this user does not know if the battery in operation is running low in power and should be replaced or recharged immediately. Further, these prior-art batteries are each composed of an assembly of metal-air cells connected in series (e.g., in Mathews, et al., U.S. Pat. No. 4,177,327) and they do not address the issues of timing at which an individual cell assembly is actuated.
A particularly promising approach to the reduction of anode passivation and self-discharge problems and, hence, much enhanced battery operating life and better utilization of the anode material has been developed by the present applicant (W. C. Huang) and his colleague J. Liu (“Metal-Air Battery with an Extended Service Life,” U.S. Pat. Pending, (Ser. No. 10/105,495) Mar. 26, 2002). This approach entails constructing a battery that has a control circuit and a plurality of metal-air cell assemblies that are electronically connected in parallel. Each cell assembly comprises a casing with a controllable air vent thereon and at least a metal-air cell inside the casing. The controllable air vent is closed during a battery storage period and is opened in response to a programmed signal in order to allow outside air to enter the assembly through the air vent to activate the operation of the corresponding cell assembly. The control circuit, including a sampling unit, a power control unit and a logic control unit, is capable of sensing the battery output voltage and sending programmed signals to open or close up the air vents at the same time or at different moments of time in a programmed fashion. This actively controlled battery has an exceptionally long operating life. In this approach, however, the anode was in physical contact with the electrolyte in each metal-air cell when the battery was fabricated. Anode corrosion could still occur in this type metal-air battery where the outside air was admitted when needed (hereinafter referred to as the “air admittance on demand” strategy).
Therefore, it is an object of the present invention to provide a smart battery that is composed of a multiplicity of metal-air cell assemblies that can be separately actuated in a programmed-timing fashion. Such a programmed-timing fashion could include a sequential fashion, in which the cell assemblies are actuated or switched on one after another. The actuation of a cell assembly is accomplished by turning on its corresponding electrolyte control valve in order for the anode in individual metal-air cells to receive the needed electrolyte (hereinafter referred to as “anode-electrolyte contact on demand” strategy).
Another object of the present invention is to provide a smart metal-air battery that makes use of both the “air admittance on demand” and “anode-electrolyte contact on demand” strategies.
It is still another object of the present invention to provide a battery that exhibits little or no anode passivation, self-discharge, current leakage, and/or anode corrosion problems.
A specific object of the present invention is to provide a metal-air battery that has a long storage life and a long operating life.